1. Field of the Invention
The present invention relates in general to an earth leakage breaker for sensing an electric leakage caused on an alternating current (referred to hereinafter as AC) line and breaking the sensed electric leakage, and more particularly to an earth leakage breaker for detecting a current difference on the AC line with respect to both positive (+) and negative (-) directions to sense and break accurately the electric leakage of the AC line and prevent a faulty operation due to a high surge resulting from a flash of lightning.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional earth leakage breaker. As shown in this drawing, the conventional earth leakage breaker comprises a zero-phase-sequence current transformer ZCT for detecting a current difference on an AC line to sense an electric leakage of the AC line, a comparator 1 for comparing an output voltage from the zero-phase-sequence current transformer ZCT with a trigger reference voltage VT, a level discriminator 2 for discriminating a level of an output signal from the comparator 1, a duration generator 3 for generating a duration in response to an output signal from the level discriminator 2, a pulse width generator 4 for generating a pulse width corresponding to the duration from the duration generator 3, and a trigger circuit 5 for breaking the AC line by the pulse width from the pulse width generator 4.
The operation of the conventional earth leakage breaker with the above-mentioned construction will hereinafter be described with reference to FIGS. 2A to 4D. FIGS. 2A to 2D are waveform diagrams of the signals from the components in FIG. 1 in the case where the electric leakage is caused on the AC line. FIGS. 3A to 3D are waveform diagrams of the signals from the components in FIG. 1 in the case where a positive (+) high surge is applied to the AC line. FIGS. 4A to 4D are waveform diagrams of the signals from the components in FIG. 1 in the case where a negative (-) high surge is applied to the AC line.
First, in the case where the electric leakage is caused on the AC line, a leakage current is generated on the AC line. The generated leakage current on the AC line is detected by the zero-phase-sequence current transformer ZCT, which then provides the resultant output voltage as shown in FIG. 2A. The comparator 1 compares the output voltage from the zero-phase-sequence current transformer ZCT with the trigger reference voltage V.sub.T and outputs the resultant signal to the level discriminator 2.
The level discriminator 2 discriminates the level of the output signal from the comparator 1. FIG. 5 is a detailed circuit diagram of the level discriminator 2. As shown in this drawing, the level discriminator 2 includes two constant current sources I1 and I2 connected in parallel to an output terminal of the comparator 1 and in series between a supply voltage source Vcc and a ground terminal, and a capacitor C1 connected between a node of the two constant current sources I1 and I2 and an output terminal of the level discriminator 2 and the ground terminal. If the output signal from the comparator 1 is low in level, the constant current source I2 is driven to discharge a voltage from the capacitor C1 to the ground terminal. As a result, the output signal from the level discriminator 2 becomes low (0 V) in level. On the other hand, if the output signal from the comparator 1 is high in level, the constant current source I1 is driven to charge a voltage on the capacitor C1. As a result, the output signal from the level discriminator 2 becomes high in level. In other words, the output signal of the level discriminator 2, as shown in FIG. 2B, is "high" only at those regions where the ZCT output reaches the trigger reference voltage V.sub.T. Then, the output signal from the level discriminator 2 is applied to the duration generator 3.
Upon receiving the output signal from the level discriminator 2, the duration generator 3 generates the duration, which is present between threshold voltage levels Vth1 and Vth2 as shown in FIG. 2C. Then, the duration generator 3 outputs the resultant signal to the pulse width generator 4. The pulse width generator 4 generates the pulse width as shown in FIG. 2D corresponding to the duration from the duration generator 3. Noticeably, the pulse width generator 4 generates no pulse width if the level of the output signal from the level discriminator 2 is not arrived at the threshold voltage level Vth1. Then, the generated pulse width from the pulse width generator 4 is supplied to the trigger circuit 5.
Then, the trigger circuit 5 breaks the A C line by the pulse width from the pulse width generator 4. As a result, the electric leakage on the AC line is broken. In the conventional earth leakage breaker, if a negative (-) high surge is present on the AC line, such surge is properly recognized as electric leakage and proper breaking of the AC line occurs. However, when a positive (+) high surge is present on the AC line, such surge is erroneously recognized as electric leakage as shown in FIGS. 3A to 3D, thus resulting in faulty operation due to unwanted breaking of the AC line. However, the above-mentioned conventional earth leakage breaker has a disadvantage in that it senses the negative (-) high surge applied to the AC line as the electric leakage as shown in FIGS. 4A to 4D, resulting in the faulty operation being caused. Also, the conventional earth leakage breaker does not detect the current difference on the AC line with respect to a positive (+) direction even when the electric leakage is caused in the positive (+) direction on the AC line. For this reason, the conventional earth leakage breaker cannot break accurately the electric leakage of the AC line.